remove

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 380
      def remove(conn)
        needs_new_connection = false

        synchronize do
          remove_connection_from_thread_cache conn

          @connections.delete conn
          @available.delete conn

          # @available.any_waiting? => true means that prior to removing this
          # conn, the pool was at its max size (@connections.size == @size).
          # This would mean that any threads stuck waiting in the queue wouldn't
          # know they could checkout_new_connection, so let's do it for them.
          # Because condition-wait loop is encapsulated in the Queue class
          # (that in turn is oblivious to ConnectionPool implementation), threads
          # that are "stuck" there are helpless. They have no way of creating
          # new connections and are completely reliant on us feeding available
          # connections into the Queue.
          needs_new_connection = @available.any_waiting?
        end

        # This is intentionally done outside of the synchronized section as we
        # would like not to hold the main mutex while checking out new connections.
        # Thus there is some chance that needs_new_connection information is now
        # stale, we can live with that (bulk_make_new_connections will make
        # sure not to exceed the pool's @size limit).
        bulk_make_new_connections(1) if needs_new_connection
      end